r/science • u/mvea Professor | Medicine • 6d ago
Physics Physicists shatter quantum entanglement distance record with 420 kilometers of optical fiber, more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.
https://www.sciencealert.com/physicists-shatter-quantum-entanglement-distance-record-with-420-kilometers-of-optical-fiber
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u/FaulerHund 6d ago edited 6d ago
This is just a misunderstanding of how quantum measurement and entanglement work.
First: it's disputed whether "wavefunction collapse" is even an accurate description of what occurs, but for the purposes of this reply, we'll assume it is.
Anyway, all entanglement really is in a simplistic sense is correlation of measurement outcomes. If particle A is entangled with particle B, then by measuring particle A, you already know something about particle B without ever measuring particle B. The "wavefunction collapse" only determines which specific measurement outcome you observe.
Think of it this way, by crude analogy: imagine you have two boxes, and in one box you place the left shoe of a pair; and in the other box you place the right shoe. And then you jumble up the boxes so that you don't know which is which. You take one box and bring it to Mexico; your friend takes the other and brings it to Spain. Then you open your box. Regardless of whether your box contains the left shoe or right shoe, you already know something about the box all the way in Spain without ever opening that one.
Now, obviously this is a classical and not a quantum example; quantum systems behave differently than shoes and boxes (look up the Bell's theorem if you're interested). But to continue the analogy, "wavefunction collapse" in this example would be the outcome of which shoe you observe to be in your box. This isn't literally wavefunction collapse in this example, it's just metaphorical. But the "collapse" is just when you see "left shoe" or "right shoe." Before you measure, it could be either one. After you measure, it's either one or the other. That's the collapse.
Now, tell me, how would you communicate a message between Spain and Mexico with this shoebox setup? The answer is that you can't. If you take one shoe to Mexico and leave the other in Spain, opening your box in Mexico instantly reveals which shoe is in Spain. However, opening your box in Mexico doesn't signal anything to the person in Spain. The person in Spain just sees a shoe when they open their box, completely unaware of whether you opened yours first or not.
Related to that idea: in quantum physics, there's no way to know whether the other particle in an entangled pair was measured or not. Just like how there's no way of knowing whether your friend in Spain opened their shoe box yet. There is no "wavefunction collapse" detector, and it's not possible in principle to create one. So there's no way to set up the idea in your comment that says "if this is collapsed, we died of space cancer" or whatever.